The invention is directed to an electrostatic precipitator for removing entrained particles from a gas stream by passing the particle through an intense high current electric field arranged in a thin plane to quickly electrically charge the particles for collection on a collector electrode. It is more specifically directed to an electrostatic precipitator for efficiently removing dust particles from an air stream by the use of two or more collector stages. The primary stage provides a charger and collector electrode suitably positioned in a plane transverse to the gas flow. The collector electrode include cooling or heating of the electrode to control the resistivity of the dust particles retained on the collector electrode to prevent back ionization in the primary stage to allow a high current electrical field to greatly improve the efficiency of the operation of the precipitator.
The present day electrostatic dust precipitator, in most cases comprises a pair or pairs of electrodes which are strategically placed in the path of an air or gas stream which includes dust particles. The purpose is to remove the dust particles and to prevent their being transported with the gas to be vented through the chimney or stack to the atmosphere. In order to reduce this pollution and efficiently remove the particles from the gas stream the electrodes are positively and negatively charged by a high voltage direct current which is applied between electrodes. In this way, a very strong electric field is generated between the corona discharge electrode which can be a wire and the opposite electrode.
When the gas stream to be treated, which can be smoke or dust laden air, is directed past the space between the charged electrodes, the dust particles in the gas are negatively charged and then attracted to the positively charged dust collecting electrodes. This arrangement has been known for many years to be highly efficient for dust precipitation and is commonly used in power generating plants. In the past it has been a well known practice to use rods or wires as the ionizing electrode and pipes or flat tubes plates as the collector electrode.
The dust particles adhere and collect on the collector electrode and during operation and dust must be periodically removed from the electrodes to retain desired high particle removal efficiency. Various mechanisms have been provided in the past to facilitate the removal of these dust particles, such as by mechanical hammer apparatus which tap or shock the electrode structure to establish vibrations which cause dust particles to fall by gravity to the bottom of the duct. Sometimes it is necessary to temporarily remove the voltage from the electrodes to eliminate the electric field during the dust removal process.
Another method for removing the dust from the collectors has been the use of mechanical scrapers which periodically are moved up and down along the outer surfaces of the collector electrodes causing the electrodes to be physically scraped so as to remove the collected dust from the electrodes. Although the shock or hammer method creates some stress and possible eventual cracking in the electrode structures, it has still been found to be the easiest and simplest way of removing the dust from the collectors. The scraping arrangement requires extensive mechanisms and power units for raising and lowering the scrapers with additional provisions required for maintaining the scraper in contact with the surface of the electrodes during the cleaning process. In some of these precipitators the electrodes are arranged as flat plates so that the scraper will be able to move along an extended flat surface to facilitate the removal. The scrapers have been found to have greater disadvantages which render them less desirable than the shock method for cleaning or removing the dust particles.
It has been well known in the prior art that in order to improve the dust removing capabilities of the precipitator, it is necessary to provide a high current electrical field and still eliminate or prevent the presence of back ionization in the vicinity of the collector electrode. Several methods have been suggested for controlling this phenomena. One of these is the control of the resistivity characteristics of the particles by controlling the temperature of the electrodes. Various arrangements have been suggested for controlling this temperature such as passing a cooling fluid through the electrodes during operation but in most cases, this required a considerable amount of cooling or heating capacity required to efficiently maintain the temperature control of the collectors because of the rather large cross-sectional areas required for the electrodes. Until recently the requirements for capacity in the cooling or heating of the electrodes have necessarily limited the use of this method to small air conditioning systems and experimental use. Temperature control has now come into more widespread use and is now being found to provide the desired control for the collector electrodes.
The present invention provides a new and unique arrangement for greatly reducing the size of the electrodes and thus the quantity of cooling or heating fluid required to provide the necessary control of the resistivity of the dust particles. The cost and size of the auxiliary equipment required for operation of the present precipitator can be greatly reduced and provide a substantial improvement in the electrostatic precipitator according to the present invention. In addition, it has been found that by the use of the present electrode geometry and structure the collector electrodes can still be rapped or hammered in the conventional manner without the detrimental effects found in the larger sized units.
The small size of the unit not only reduces the cost and capital equipment requirements but it also represents the optimum design for heat transfer (cooling) of the particles. The warmest area of collected particles in the charging section will have the highest resistivity and will limit the operating voltage. The effectiveness of the system will depend upon how well the collected particles are cooled. Therefore, it is necessary to eliminate any fins or extending surfaces from the electrodes where particles will collect and not be cooled sufficiently.